Physical activity lowers the risk of obesity in children, is independently associated with numerous benefits related to health and well-being, and tracks from preschool-age to adolescence. The amount of time preschoolers spend outdoors is thought to correlate with their physical activity levels. Yet, most US children today likely spend less time playing outdoors than previous generations. The study of children's outdoor play time is not merely about resurrecting less digital, more unstructured childhoods, but is related to serious public health issues. Encouraging active outdoor play may prove to be an important method of enhancing children's overall development and health. Since the majority of US preschool-age children spend considerable time in child care programs, examining and pursuing strategies to increase outdoor physical activity in these settings is critical. Through child care settings, we may be able to change behaviors in large groups of children and particularly benefit more vulnerable children who have fewer opportunities for outdoor play and physical activity at home. Studying how physical activity and outdoor play enhance learning and development may provide additional incentives for early childhood professionals, parents and policy makers to promote these activities. An understanding of the accurate relationship between outdoor time and various outcomes is currently hampered by poor measurement. Robust measurement of outdoor time and an understanding of the context within which children are more active would inform further research and intervention. The overall research goal of this proposal is to characterize outdoor physical activity for preschoolers at child care and to understand environmental and social determinants of outdoor physical activity in order to inform intervention development. Specifically, Aim 1 is to validate light sensors and GPS units against direct observation as accurate measures of outdoor time. Aim 2 is to accurately quantify preschoolers' outdoor time at child care and its relationship to accelerometer-measured physical activity and various modifiable child care environment characteristics. Aim 3 focuses on developing and piloting an intervention in child care programs to increase the quality and quantity of outdoor time and studying the resulting impact on physical activity, health and development outcomes. This research plan is part of a K23 proposal designed to provide the scholarly training, mentorship and support necessary for Dr. Tandon to gain skills and experience needed to conduct research on children's outdoor physical activity to then develop and translate effective interventions into practices, programs and policies. Dr. Tandon is a pediatrician whose career goal is to become an independent investigator with a focus on promoting physical activity in children to address the critical health problems associated with sedentary, indoor lifestyles. To achieve this objective, she will pursue training in: 1) the measurement of physical activity and outdoor time, 2) the relationship of the built environment to physical activity, 3) incorporating cognitive outcomes into physical activity research and 4) developing and testing community-based interventions. .